A system which transmits an engine output of a vehicle to a vehicle drive shaft via a torque converter or fluid coupling and a continuously variable transmission is disclosed for example in Tokkai Sho 61-105353 published by the Japanese Patent Office in 1986.
In this system, a target speed change ratio is set based on engine running conditions such as engine load and vehicle speed, and feedback control is performed so that the speed change ratio of the continuously variable transmission coincides with a target speed change ratio.
In this continuously variable transmission, a drive pulley and driven pulley are connected by a V- belt, the contact radius between the V-belt and the pulleys being varied by varying the groove width of the pulleys in contact with the V-belt. The groove width of the pulleys varies according to an oil pressure, so the speed change ratio can be varied by controlling the oil pressure. Oil pressure control is performed via a step motor which responds to an output signal from a control unit comprising a microcomputer, and an oil pressure control valve driven by the step motor.
In such a device, the ability of the real speed change ratio to follow or track the target speed change ratio, is determined by the response characteristics of a speed change ratio varying mechanism comprising the step motor and oil pressure control valve, but the desired tracking characteristics are not necessarily obtained.
Therefore, in a transient state when the speed change ratio is varying, overshoot may occur wherein the real engine speed exceeds the target engine rotation speed, and fuel consumption of the engine is adversely affected by this overshoot.